3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a down link communication method. With 3GPP-LTE, a radio communication base station apparatus (hereinafter “base station”) transmits RSs (reference signals) using predetermined communication resources, and radio communication terminal apparatuses (hereinafter “terminals”) perform channel estimation using received reference signals to demodulate data (see Non-Patent literature 1.) In addition, terminals use the reference signals to perform adaptive MCS (modulation and channel coding scheme) control, PMI (precoding matrix indicator) control in MIMO (multiple-input multiple-output) transmission, or received quality measurement for adaptive scheduling. Then, terminals feed obtained PMIs and received quality information (CQI: channel quality indicator) back to a base station.
In addition, when a base station has a plurality of antennas, the base station enables diversity transmission. For example, a base station transmits a plurality of data streams from a plurality of antennas (MIMO transmission) to allow high-speed transmission. In order to receive diversity-transmitted signals as described above with no error, terminals need to know the channel states from a group of antennas used for transmission in a base station, to the terminals. Therefore, RSs need to be transmitted without interfering with each other, from all antennas provided in a base station. To realize this, 3GPP-LTE adopts a method of transmitting RSs from respective antennas in a base station, using timings and carrier frequencies varying in the time domain and the frequency domain.
FIG. 1 shows a configuration of a base station having four antennas (4Tx base station) anticipated with 3GPP-LTE, and FIG. 2 shows an RS transmission method in a 4Tx base station (see Non-Patent Literature 2.) Here, in FIG. 2, the vertical axis (frequency domain) is indicated by a unit of subcarriers and the horizontal axis (time domain) is indicated by a unit of OFDM symbols. In addition, R 0, R 1, R 2 and R 3 indicate RSs transmitted from antennas 0, 1, 2 and 3 (the first, second, third and fourth antennas), respectively. Moreover, in FIG. 2, one block unit enclosed by a heavy-line frame (six subcarriers in the frequency domain and fourteen OFDM symbols in the time domain) is referred to as a resource block (RB.) Although one RB is composed of twelve subcarriers in 3GPP-LTE, the number of subcarriers constituting one RB is six here for ease of explanation. In addition, each unit of one subcarrier with one OFDM symbol constituting one RB is referred to as a resource element (RE.) As seen from FIG. 2, in order to minimize RS transmission overhead, a 4Tx base station reduces transmission frequencies of RSs (R2 and R3) from antenna 2 (third antenna) and antenna 3 (fourth antenna.)
Here, RSs shown in FIG. 2 are common to all terminals in the cell covered by a base station, and referred to as cell-specific reference signals. In addition, a base station may additionally transmit RSs multiplied by a specific weight on a per terminal basis (UE-specific reference signals) for beamforming transmission.
As described above, with 3GPP-LTE, the maximum number of antennas in a base station is four, and terminals supporting 3GPP-LTE perform data demodulation and downlink signal quality measurement, using RSs (R0 to R3 shown in FIG. 2) transmitted from a base station having maximum four antennas (4Tx base station.)
By contrast with this, LTE-advanced, which is improved 3GPP-LTE, is studying a base station having maximum eight antennas (8Tx base station.) Here, LTE-advanced needs to provide a base station complying with 3GPP-LTE in order to allow communication of terminals supporting only base stations (4Tx base stations) in 3GPP-LTE. In other words, LTE-advanced is required to accommodate both terminals supporting only a 4Tx base station (hereinafter “LTE terminals”) and terminals supporting also an 8Tx base station (hereinafter “LTE+ terminals” or “LTE-advanced terminals”).